The typical heating appliance which employs a combustible fuel has, heretofore, employed a constantly burning pilot burner to ignite fuel discharge from the main burner in response to heating demand. The increasing concern over dwindling energy resources has promoted interest in heating appliances having interrupted pilot burner operation. Past history has demonstrated a very high degree of safety and reliability in appliances utilizing standing, or continuously burning, pilots. It is the intent of interrupted pilot burner systems to equal, or surpass, such a degree of safety and reliability.
A number of control circuits, chiefly of solid state and/or digital design, have been developed to prevent the accumulation of any substantial quantity of unburnt conbustible fuel in the heating appliance. Generally these devices have employed flame responsive sensors positioned adjacent the pilot burner to generate control signals for the circuit which are indicative of the absence or presence of a pilot burner flame.
A difficulty with solid state control circuits when employed in a household appliance that is susceptible of a potentially hazardous operation is that most solid state devices have a multitude of possible failure states. Thus, although solid state devices are quite reliable in freedom from failure, the multiple manners in which these devices can fail presents a circuit designer with almost insurmountable difficulties in designing a fail-safe circuit for all potential modes of failure.
Attempts to provide an entirely fail-safe control circuit utilizing entirely solid state and/or digital devices thus results in development of control circuits of ever and ever increasing complexity, further increasing the probability of failure of the circuit.